1. Field of the Invention
The present invention relates to PN junctions and to their fabrication. More specifically, the invention concerns PN junction structures that must sustain high reverse voltages without breaking down.
2. Discussion of the Related Art
Breakdown in a PN junction is due to two different phenomena, depending on the dopant concentrations of the junction""s regions: avalanche breakdown and Zener breakdown.
If the dopant concentration of the junction""s doped regions is low, avalanche breakdown takes place. If the dopant concentration is high, Zener breakdown occurs.
In both cases, however, junction breakdown occurs when the maximum electric field reaches a critical value. The critical value of the electric field depends on the voltage applied to the junction, as well as on the dopant concentration of the doped regions thereof. To increase the breakdown voltage of a junction it is therefore necessary to decrease the dopant concentration of the doped regions thereof.
On the other hand, a lower dopant concentration makes it necessary to have thicker P and N regions, because the depletion region spreads out. The extension of the depletion region, the concentration of dopants in the silicon and the voltage applied to the junction are linked by the Poisson""s equations.
Thicker silicon regions have, however, higher on resistances.
Normally, in power semiconductor devices designed for high voltages, the junctions are of the PIN (P-Intrinsic-N) type, so as to reduce the silicon thickness and thus decreasing the on resistance value. Nevertheless, in these devices the junction area must be augmented to achieve reasonable (i.e., rather low) forward-bias voltage drops. This problem is exacerbated in unipolar conduction devices, where no modulation of conductivity exists.
An object of the present invention is to provide a PN junction structure having a high breakdown voltage.
According to the present invention, this and other objects are achieved by a PN junction structure comprising a first junction region of a first conductivity type, and a second junction region of a second conductivity type, wherein between said first and second junction regions a grid of buried insulating material regions is provided.